As is known, electronic noses are instruments which permit analysis, preferably continuous, of atmospheric air, and more in detail, make it possible to detect the components of a gas mixture, whether these components include odoriferous compounds or not, that is to say, whether these components can or cannot be detected by a human or animal olfactory system.
This specification refers to components with odoriferous compounds but without thereby limiting the scope of the invention.
Electronic noses can qualitatively classify the air analysed, assigning it to a specific olfactive class, and olfactometrically quantify the air analysed by estimating its odour concentration.
An electronic nose mimics human olfaction and, generally speaking, its operation can be divided into the following stages:                detecting the gases using suitable sensors;        processing the signals from the sensors;        recognizing the odours.        
The array of sensors is usually housed in a chamber made of a chemically inert material into which a reference gas (normally clean, i.e., odourless, air) is made to flow in order to create a reference measure for the subsequent processing of the sensor responses.
In use, the air to be analysed is delivered into the sensing chamber and produces a change in the chemical composition of the atmosphere and hence a different response from the sensors.
The measurement ends by injecting the reference gas into the sensing chamber again, cleaning the active material constituting the sensors so that the sensor response returns to the reference measurement.
In this specification, we refer to electronic noses equipped with metal oxide semiconductor (MOS) sensors but without thereby limiting the scope of the invention.
The principle on which these sensors are based is the variation in the electrical conductivity of the oxide in the presence of odoriferous substances compared to the value of this parameter under reference conditions.
During a generic measuring cycle, an electronic nose takes in the reference gas for a certain fraction of time and the air to be analysed for the remaining fraction.
Further, in prior art electronic noses, active carbons are used to clean the reference air and make it as odourless as possible.
However, electronic noses of this kind presents several disadvantages.
It is well known, for example, that the sensors they use are highly sensitive to changes in the temperature and humidity of the air to be analysed and of the reference air.
The dependence on environmental conditions makes reading of olfactive data difficult and definitely unreliable. Indeed, the temperature and humidity of the air can change considerably during the course of the day and from one season to another.
Moreover, the use of active carbons to clean the reference gas of possible contamination also tends to change the relative humidity of the gas itself.
For this reason, prior art electronic noses provide highly imprecise measurements and are unable to offer a high coefficient of repeatability.